The novel pharmacology of K-type tachykinin binding sites has been identified in several peripheral tissues. Substance K may be the mammalian endogenous ligand for this receptor whereas substance P and neuromedin K may be the endogenous ligands for P-type sites, respectively. Although all three sites are regulated identically by ions and guanine nucleotides, the distinct anatomical localization of each type of site in various tissues indicates that each is a distinct type of tachykinin peptide receptor.